Neurothekeoma and plexiform fibrohistiocytic tumor: mere histologic resemblance or histogenetic relationship?

Neurothekeoma (NTK) and plexiform fibrohistiocytic tumor (PFHT) are dermal neoplasms that share many clinical and histologic features and whose histogenesis is equally disputed. We analyzed the morphologic and immunohistochemical features of 43 NTK and 18 PFHT to evaluate a possible relationship between these tumors. On the basis of the amount of myxoid stroma, we divided NTK into myxoid neurothekeoma (MyNTK) (8 cases), mixed neurothekeoma (MiNTK) (15 cases), and cellular neurothekeoma (CNTK) (20 cases). MyNTK and MiNTK were well circumscribed lesions composed of spindle cells. CNTK had an infiltrative quality and consisted of predominantly epithelioid cells, with over 50% of the cases containing multinucleated giant cells. PFHT were predominantly composed of epithelioid cells in 11 cases, spindle cells in 1, and an even admixture of spindle and epithelioid cells in 6 cases. Their morphologic features (particularly those of the more epithelioid examples) were superimposable to those of CNTK. Immunohistochemically, there was positivity for S100 protein in 8 of 8 MyNTK, 4 of 15 MiNTK, 1 of 19 CNTK, and 0 of 18 PFHT. There was positivity for CD68 in 0 of 8 MyNTK, 6 of 15 MiNTK, 9 of 17 CNTK, and 6 of 11 PFHT. These results suggest the existence of a continuum between MyNTK and MiNTK. Furthermore, the marked morphologic and phenotypic similarities between CNTK and PFHT suggest a common histogenesis for these 2 tumors, different from that of MyNTK and MiNTK.